K个一组反转链表

// 运用反转链表II来完成
/**

• Definition for singly-linked list.

• type ListNode struct {

• Val int
  

• Next *ListNode
  

• }
  */
  func reverseKGroup(head *ListNode, k int) *ListNode {
  cur := head
  len := 1
  for head != nil {
  head = head.Next
  len++
  }

  slow, fast := 1, k

  res := cur
  for fast < len {
  res = reverseBetween(cur, slow, fast)
  slow = fast + 1
  fast += k
  cur = res
  }

  return res
  }

// 返回反转后的头
func reverseBetween(head *ListNode, m int, n int) *ListNode {
i := 0

var last, begin *ListNode

head = &ListNode{
    Next: head,
}
cur := head


for head != nil {
    // i 表示到访过的第几个节点
    if i > m {
        if i <= n {
            last.Next = head.Next
            head.Next = begin.Next
            begin.Next = head

            head = last // head需要更新为last，注意这里很重要，对于链表反转来说
        } else {
            break
        }
    }

    if i == m {

// head为当前节点，last为上一个
// begin表示反转的头的上一个
// last为反转的尾
begin = last
}

    last = head
    // 增加到访过的节点
    head = head.Next
    i++
}

return cur.Next

}